1. Field o f the Invention
The present invention relates generally to an in-space thruster system and, more particularly, to a system utilizing a pair of electric thrusters which co-operate for orbit raising, for north-south station keeping when the desired orbit is achieved, and for selectively unloading momentum wheels used for controlling the orientation of the spacecraft.
2. Description of the Prior Art
Satellites or spacecraft orbiting the earth are useful for many applications including weather data collection and communications. As these applications have become more complex, they have resulted in a demand for more powerful payloads and hence more massive spacecraft. However, heavier spacecraft are increasingly more difficult and expensive to place, and then maintain, on orbit.
A typical spacecraft is placed on orbit by a combination of a launch vehicle and its own propulsion systems. A launch vehicle will propel and release the spacecraft in an initial lower orbit about the earth; once in this initial lower orbit the spacecraft propulsion system will be responsible for propelling the spacecraft to its final orbit.
A launch vehicle will have a limited lift capability, beyond which, it will not be capable of delivering the spacecraft to an acceptable orbit. The lift limit is the maximum spacecraft separation mass, i.e., the sum of the spacecraft's fuel and dry mass. Generally, the more lift capability is required, the larger and more expensive is the launch vehicle. Thus as the mass of a spacecraft increases during the design process, the availability of the less capable, inexpensive launch vehicles decreases. There is a real desire to maintain compatibility with a broad range of less capable and inexpensive launch vehicles as spacecraft dry mass increases.
Clearly, if the dry mass of a spacecraft increases, then its fuel mass must decrease to remain compatible with inexpensive launch vehicles. The fuel mass can decrease if the propulsion subsystem becomes more efficient. At the present, liquid chemical thrusters are the propulsion means of choice on most spacecraft for propelling the spacecraft during its transfer orbit to final on-station orbit, a process referred to herein as orbit raising. The mass of the chemical propellant needed for this maneuver can be as much as half of the separated mass. Dry mass could be increased by, say, 50% if the need for half of this fuel could be eliminated. Another inefficiency associated with prior art orbit raising is that to achieve a high efficient large chemical thrust, a dedicated main satellite thruster (MST) is required. This thruster is too powerful to be used for the delicate maneuvers required on orbit. Clearly, there is need for a more efficient propulsion system using less fuel mass during orbit raising.
Once on station the propulsion system is responsible for maintaining the orbit throughout the life of the mission. Commonly, spacecraft orbit the earth at the same revolution rate as the earth spins. These spacecraft and corresponding orbits are referred to as "synchronous" or "geosynchronous". When the synchronous orbit lies in the plane of the earth's equator, the synchronous spacecraft is also called geostationary and operates within a "stationary" orbit. It is generally well known in the art that various forces act on synchronous spacecraft which move the spacecraft out of a desired orbit. These forces are due to several sources including the gravitational effects of the sun and moon, the elliptical shape of the earth, and solar radiation pressure. To counter these forces, synchronous spacecraft are equipped with propulsion systems that are fired at intervals in order to maintain station at a desired geostationary and longitudinal location. This maintenance requires control of the inclination, eccentricity, and drift of the spacecraft. The orbit's inclination defines the north-south position of the spacecraft relative to the earth's equator. Eccentricity is the measure of the noncircularity of the spacecraft orbit. That is, the measure of the variation of the distance the spacecraft is from the center of the earth as the spacecraft moves around its orbit. Drift is the measure of the difference in longitude of the spacecraft's subsatellite point and the desired geostationary longitude as time progresses.
Current three-axis stabilized spacecraft use liquid chemical propulsion for station keeping. Typically, one set of thrusters are used for controlling the inclination while a second set is used for controlling the drift and eccentricity. Of these maneuvers, controlling inclination, commonly referred to as north-south station keeping, requires the most fuel. A spacecraft with a dry mass of 2000 kg will require over 400 kg of liquid propellant on-station for a 12 year mission. Additional fuel is also consumed by the MST just to place this 400 kg of fuel on orbit. Clearly, there is need for a more efficient use of fuel during north-south station keeping maneuvers.
Once on-station, a spacecraft must maintain its attitude in addition to its orbital position. This orbital maintenance is essential for geosynchronous communications spacecraft in which communication hardware must be pointed to a preselected planetary location. Disturbance torques, such as solar pressure, work to produce undesired spacecraft attitude motion. Momentum wheel stabilization systems are commonly used to counteract such disturbance torques. Such systems typically include one or more momentum wheels and control loops to sense and control changes in the spacecraft attitude. Sensors on the spacecraft may detect yaw, pitch and roll. The control loops determine the required speed of the wheels to absorb or off-load momentum based on the sensed attitude. Momentum stored in the momentum wheels must be periodically relieved, desaturated, or unloaded, to keep the momentum wheels within a finite operable speed range. Desaturation is typically accomplished by applying an external torque to the spacecraft through propulsion thrusting. This requires more fuel and more thrusters. An efficient propulsion system would maximize the efficient use of the fuel and minimize the number of thrusters needed for these maneuvers.
In summary, the prior art geosynchronous satellite may have a dozen or more small liquid chemical thrusters and a large MST and will require more mass in fuel than mass in payload and supporting structure. Recent developments in the art have been directed toward reducing this proportion of fuel mass. One significant development is the electric thruster. In one type of electric thruster, a plasma thruster, xenon atoms are ionized in collisions with electrons creating xenon ions. Thrust is created as the charged xenon ions are accelerated out of the thruster by an electric-magnetic field. Although there is an initial weight penalty for the electric o propulsion system hardware, the specific impulse (The measure of thruster efficency) of electric thrusters is substantially higher than chemical systems and can lead to a net savings in propulsion system mass. The higher specific impulse of the electric thruster (approximately 1500-3000 seconds compared to 300 seconds for chemical thrusters) corresponds to a larger change in spacecraft velocity or momentum per unit of consumed fuel. Thus less propulsion system mass is needed for a given spacecraft dry mass.
The thrust of the MST is several hundred Newtons and its total orbit raising impulse can be delivered in a few hours. The electric thruster could potentially reduce the fuel mass needed during orbit raising. However, the thrust of an electric thruster is very small, measured in mili-Newtons and its total impulse takes many days to deliver. Hence orbit raising purely with electric thrusters would require many thrusters, take a long time and expose the spacecraft to the Van Allen radiation belts for long periods of time damaging the solar arrays. Further one skilled in the art would have a difficult time determining electric thruster mounting locations on the spacecraft which would be useful for electric orbit raising and would at a later time be in a useable position for on-station maneuvers. What is needed is an electric thruster system that can supplement the chemical system during orbit raising to get to orbit in a timely fashion yet show a substantial fuel mass savings and still be useful on station.
For north-south station keeping, it is desirable to orient thrusters along the north-south axes of the spacecraft. However, there are several obstacles to orienting the chemical or electric thrusters along the north-south axes. Because the location of the solar arrays along or near the north-south axis, a thruster must be offset from the north-south axis to avoid plume contamination of the arrays. If the offset thruster is placed with zero cant, its thrust will not extend through the spacecraft center of mass; this would produce a torque inducing a spacecraft rotation. To combat this torque, an additional thruster is needed (this is in fact done in chemical thruster station keeping). Thus a minimum of four chemical thrusters are used in the prior art providing both north and south thrust. FIG. 1A shows prior art locations of four chemical thrusters 2 on a spacecraft 4 with solar arrays 6 aligned along the north-south direction. These are typical locations for north and south thrusting thrusters. FIG. 1A also shows a plurality of other thrusters 7 used for other maneuvers. A second problem with placing thrusters with zero cant along the north and south directions is that thruster plume impinges on the solar array panel degrading array performance as well as producing spacecraft torque disturbances. This is especially a problem with electric thrusters which have wide plumes. Further, the electric thruster plume can interfere with RF communication and should not be mounted near the nadir (earth facing) deck 8 which contains RF communication hardware (antennas) 10.
The prior art does show on-station use of electric thrusters. This configuration removes the need for the four chemical thrusters 2 shown in FIG. 1A. U.S. Pat. No. 5,020,745 issued Jun. 4, 1991 to Anzel describes an electric thruster arrangement and its use for north-south station keeping of a three-axis stabilized spacecraft although it does not indicate whether it has any utility for orbit raising or unloading momentum. The arrangement makes use of two thrusters as illustrated in FIG. 1B. The thrusters are mounted on the anti-nadir face 48 of the satellite. A single north thruster 12 is canted at an angle .theta. from the north-south axis of the satellite providing thrust in a southerly direction and a single south thruster 14 is canted at the angle .theta. from the north-south axis providing thrust in a northerly direction. The cant angles are chosen such that the thrusters' thrust vectors are directed through the center of mass 16 of the spacecraft so that their thrust does not provide an attitude disturbance torque. Thus each thruster provides thrust with a component along the desirable north-south direction and a component in an undesirable radial direction. By splitting the station keeping maneuver into two burns, a north thruster burn followed twelve hours later by a south thruster burn, the effects of the undesirable radial components of thrust cancel and the net effect is a desirable station keeping maneuver. However, since the thrusters are continually directed toward the center of mass, Anzel's invention cannot purposely produce disturbance torques to unload momentum wheels or otherwise adjust spacecraft attitude.
In addition, Anzel's placement of the thrusters on the anti-nadir face 48 results in very large cant angles .theta.. This means that a great portion of thrust is in the radial direction and not along the north-south axis. This corresponds to wasted fuel and increased spacecraft separation mass. The fuel efficiency for north-south station keeping is reduced by the cosine of the cant angle .theta. between the thrust vector and north-south axis; the following is an example of extra fuel usage versus angle, assuming a typical total of 107 kg of fuel for electric north-south station keeping with no cant angle:
______________________________________ Angle (degrees) % Additional Fuel Usage Delta Mass (kg) ______________________________________ 0 0 0 30 15.5 16.6 35 22 23.5 40 30.5 32.64 45 41.4 44.3 50 55.6 59.5 55 74.3 79.5 60 100 107 ______________________________________
Therefore large cant angles can nearly double the amount of required station keeping fuel. In addition to extra mass, this also means that the time of a given maneuver increases causing unnecessary wear on the thrusters. This extra thruster usage is highly undesirable and a fundamental problem with the partial solution of Anzel.
U.S. Pat. No. 5,349,532 issued Sep. 20, 1994 to Tilley et al. discloses a technique for enhancing efficiency and reliability over then-known techniques by simultaneously stabilizing attitude dynamics and desaturating the momentum wheel system of the spacecraft while performing north-south station keeping maneuvers. In order to achieve these results the spacecraft position attitude and stored wheel momentum are sensed. The forces necessary to perform station keeping maneuvers, the torques required to produce the desired attitude for the spacecraft, and desaturate the wheels are determined and ion thrusters are throttled and gimbaled to produce the desired torques on the spacecraft. However, the system relies on placing thrusters near the nadir or earth deck as well as the anti-earth deck which is not optimum with electric thrusters. Although the system has many advantages, potential remains for electric thruster plume impingement on the solar arrays and RF equipment near the earth-deck. Further this patent does not address orbit raising.
The prior art, therefore, is lacking a highly efficient electric propulsion system which uses the same thrusters for orbit raising as on-station station keeping, allows a spacecraft of substantial dry mass to be launched on less capable launch vehicles, allows for spacecraft separation mass to be over 50% dry mass, performs north-south station keeping with a small efficient cant angle, can unload momentum wheels, and avoids plume interference with solar array and RF communication hardware near the earth-facing side of the spacecraft.
It was with knowledge of the foregoing that the present invention was conceived and has now been reduced to practice.